1. Field of the Invention
The invention relates to optical storage media and fabrication method thereof. More particularly, the present invention relates to optical storage media having three-dimensional data pattern and fabrication method thereof.
2. Description of the Related Art
In optical storage media, digital information is usually written as local variations in thickness, refractive index or absorption coefficient. Optical storage media are more convenient than magnetic, because of less restriction of parallel writing or reading of information, and are usually formed as optical discs suitable for read-only or recordable operations.
However, since conventional optical storage media store data in a two-dimensional data pattern, ultimate pixel capacity thereof is restricted by the light diffraction limit. Although quadrupled capacity can be achieved by a measure of super resolution at fractions of wavelength to store 3–5 bits in a single pit, implementation of this measure demands very precise and sophisticated optical, mechanical and electronic equipment in a high quality medium, which obviously makes this approach expensive and less feasible.
To obtain larger capacity of optical storage media, three-dimensional (3D) data pattern (volume holographic storage), further utilizing a depth direction, has been disclosed. It is obvious that 3D recording can dramatically increase the storage capacity of the device. There are known 3D recording methods, based, for example, on 3D volume storage by virtue of local changes in the refractive index of optical media. It has been proposed that by writing and reading data in a 3D format, data storage densities greater that 1012 bits cm3 can be achieved.
Recording methods are based on 3D volume storage utilizing local changes in refractive index of optical media. These local variations in refractive index result in birefringence and variations in polarization of the reading beam transmitted through the media. The variations are detectable and can be interpreted as binary code.
However, due to the diffraction and power loss caused by the multiple layers, the fluorescent signal therefrom weak enough that high power lasers and highly sensitive detectors are required for detection thereof.
To overcome the drawbacks described, a 3D optical storage medium with a signal-amplifying structure has been disclosed. FIG. 1 is a cross section of the structure of a 3D optical storage medium. The 3D optical storage medium 10 comprises two data layers 20 and an isolation layer 40. Each data layer 20, located between an upper electrode 30 and a lower electrode 32, comprises a plurality of pits 22 filled by an active layer 24. The adjacent upper and lower electrodes are separated by the isolation layer 40. Moreover, the 3D optical storage medium 10 further comprises a signal-amplifying structure 80, comprising a upper electrode 34, a photoconductive layer 60, an electroluminescent layer 70, and a lower electrode 36, bonded in that order.
A voltage is applied to signal-amplifying structure 80 through the electrodes 34 and 36. When the data layer 20 is read by laser beams, the data layer 20 emits a weak excitation light into the photoconductive layer 60 inducing a photocurrent therein. The photocurrent leads to a redistribution of the voltage between the photoconductive layer 60 and the electroluminescent layer 70, thereby causing a reliable electroluminescence signal. Hence, the 3D optical storage medium 10 can efficiently amplify signal strength through the signal-amplifying structure 80.
However, due to the additional formation of signal-amplifying structure 80, the 3D optical storage medium 10 has a high cost and is difficult to fabricate. Furthermore, since an input voltage must be applied to the 3D optical storage medium 10 to implement the signal-amplifying structure 80. Compatibility between the 3D optical storage medium 10 and the usual optical media drive can thus be a problem, causing inconvenience and difficulty.
A 3D optical storage medium employing a small molecular fluorescent material has also been disclosed to simplify the structure and fabrication process thereof. Referring to FIG. 2, the 3D optical storage medium 100 comprises a substrate 110, a plurality of information layers 120, and a plurality of adhesive layers 130, wherein each information layer 120 has a plurality of pits filled with a small molecular fluorescent material, such as nile blue, rhodamine, cyanine, acridine, and phenoxazone. Due to the considerably lower quantum yield of small molecular fluorescent materials, a photoconductive material is further doped into the small molecular fluorescent material to enhance the signal strength, thereby increasing the process complexity of 3D optical storage media. Furthermore, the mentioned small molecular fluorescent material described exhibits an absorption wavelength of 580 to 650 nm with the stock shift (difference between the absorbed wavelength and the emitted wavelength) thereof less than 30 nm, resulting in crosstalk and a decrease in the signal-to-noise (S/N) ratio. Moreover, since the small molecular fluorescent material has to be dispersed in a polymer material to avoid concentration quenching effect, the cost and process complexity of 3D optical storage media are increased.
Therefore, in order to meet the demands of the market, it is necessary to develop a 3D optical storage medium with a simple manufacturing process and structure to provide improved the recording sensitivity thereof.